This invention relates to fiber gratings, and more particularly to a tube-encased fiber grating.
It is known in the art of fiber optics that Bragg gratings embedded in the fiber may be used to sense parameters such as temperature and strain, such as is described in U.S. Pat. No. 4,806,012, entitled xe2x80x9cDistributed, Spatially Resolving Optical Fiber Strain Gaugexe2x80x9d, to Meltz et al, and U.S. Pat. No. 4,996,419, entitled xe2x80x9cDistributed Multiplexed Optical Fiber Bragg Grating Sensor Arrangementxe2x80x9d, to Morey, which are incorporated herein by reference to the extent necessary to understand the present invention. It is also known that fiber gratings may be used in compression to act as a tunable filter or tunable fiber laser, as is described in U.S. Pat. No. 5,469,520, entitled xe2x80x9cCompression Tuned Fiber Gratingxe2x80x9d to Morey, et al and U.S. Pat. No. 5,691,999, entitled xe2x80x9cCompression Tuned Fiber Laserxe2x80x9d to Ball et al., respectively, which are incorporated herein by reference to the extent necessary to understand the present invention.
However, when a fiber grating by itself, is compressed, the fiber buckles. One technique used to avoid fiber buckling is to use sliding ferrules around the fiber and grating and to place the ferrules in a mechanical structure to guide, align and confine the ferrules and the fiber. Such a technique is described in the aforementioned U.S. Pat. Nos. 5,469,520 and 5,691,999. However, it would be desirable to obtain a configuration that allows a fiber grating to be compressed without buckling and without sliding ferrules and without requiring such a mechanical structure.
It is also known that Bragg gratings in optical fibers may create unwanted coupling between the core and cladding modes of a fiber. The larger the mode field overlap between the two modes, the larger the coupling. Such coupling creates unwanted optical losses in the fiber.
Objects of the present invention include provision of a fiber grating configuration that allows the grating to be used in compression without requiring sliding ferrules or a mechanical supporting structure and/or that is suitable for reducing core to cladding coupling.
According to the present invention a tube-encased fiber optic Bragg grating, comprises an optical fiber, having at least one Bragg grating embedded therein; and a tube, having the optical fiber and the Bragg grating encased therein along a longitudinal axis of the tube, the tube being fused to at least a portion of the fiber at a location where at least a portion of the Bragg grating is located.
According fiurther to the present invention, the tube is made of a glass material. According flirther to the present invention the tube is fused to the optical fiber on opposite axial sides of the Bragg grating.
The present invention provides a fiber grating encased in and fused to at least a portion of a capillary tube and a method for making same. The tube may be made of a glass material for encasing a glass fiber. The encased grating allows the grating to be compressed without buckling the fiber. Also, it allows the grating to be strain isolated from strains elsewhere on the fiber. The invention may also be used in numerous applications where fiber grating compression may be used, e.g.,parameter sensing or wavelength tuning. Also, the invention exhibits lower mode coupling from the fiber core to the cladding modes due to the effective increased diameter of the cladding where the tube is fused to the fiber where the grating is located.
The grating may be embedded (or imprinted) in the fiber before or after the fiber is encased in the tube. To cause the fiber to become encased in and fused to the tube, the tube may be heated and collapsed around the fiber.
Also, one or more gratings, fiber lasers, or a plurality of fibers may be encased in the tube. The grating(s) or laser(s) are xe2x80x9cencasedxe2x80x9d in the tube by having the tube fused to the fiber on the grating area and/or on opposite axial ends of the grating area adjacent to or a predetermined distance from the grating. The grating(s) or laser(s) may be fused within the tube or partially within or to the outer surface of the tube.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.